Gardaí have arrested two more men in connection with the abduction of Quinn Industrial Holdings executive Kevin Lunney.
Two other men, both aged 38, were arrested on Thursday morning in the Dublin area.
They are being held at Ballymun and Finglas Garda stations.
While a 25-year-old man was subsequently arrested in Dublin and is being detained under Section 50 Criminal Justice Act at Irishtown Garda station.
And a fourth man, aged 66, was arrested in Co Cavan and is being held under Section 50 Criminal Justice Act 2007 at Monaghan Garda station.
An Garda Síochána say they are continuing to appeal to any person who has any information, no matter how insignificant it appears, to contact them at Cavan on 049-4368-800, the Garda Confidential Line on 1800-666-111, Crime Stoppers on 1800-25-00-25 or any Garda station.
Mr Lunney was kidnapped and assaulted by a group of masked men in September.
He was taken from near his home in Co Fermanagh, before being dumped at the side of the road in Co Cavan.
Three people were arrested in connection with the investigation last week, and have since been released without charge.